Methoxyacetone (1-methoxy-2-propanone) provides a key structural piece in the synthetic route to metolachlor and other herbicidal compositions (see, e.g., U.S. Pat. Nos. 4,666,502 and 5,576,188). In addition, methoxyacetone has been used as a polar organic solvent, a chemical intermediate, a Schiff base reagent, an additive for cement compositions, and as an aid in the cryogenic preservation of organs.
Catalytic dehydrogenation of 1-methoxy-2-propanol (propylene glycol methyl ether) is one approach to methoxyacetone. This is generally a vapor-phase process in which the alcohol and air are fed into a hot, tubular reactor that contains a catalyst. For example, U.S. Pat. No. 3,462,495 teaches to use a "calcium nickel phosphate" catalyst and air at 425.degree. C. to convert 1-methoxy-2-propanol to methoxyacetone. Similarly, U.S. Pat. No. 4,233,246 uses air and a silver/copper catalyst at 450-700.degree. C. to effect the oxidation. U.S. Pat. No. 4,218,401 describes another vapor-phase oxidation at 225-600.degree. C. using air and a supported Group 8-10 transition metal catalyst. Copper chromite (see U.S. Pat. No. 4,141,919) has also been used as a catalyst. Unfortunately, the yield and selectivity from these catalytic dehydrogenation processes is often less than desirable.
Liquid-phase processes are also known. Chromic acid (sulfuric acid+sodium dichromate) will oxidize 1-methoxy-2-propanol (see J. Am. Chem. Soc. 71 (1949) 3558), but the yield of methoxyacetone is generally less than 30%. Mallat et al. have described liquid-phase oxidation of 1-methoxy-2-propanol using promoted, supported platinum catalysts and air as an oxidant (see, e.g., J. Catal. 142 (1993) 237 or Appl. Catal. A 79 (1991) 41.) Much earlier, Heyns and coworkers often used liquid-phase catalytic oxidation with air or oxygen and platinum on carbon to selectively oxidize secondary alcohols to ketones under mild conditions in the synthesis of carbohydrates (Angew. Chem. 69 (1957) 600).
Unfortunately, liquid-phase oxidation of 1-methoxy-2-propanol using air and a transition metal catalyst as suggested above can be challenging to practice. In our labs and under a variety of reaction conditions, including ones similar to those suggested earlier (Pt/C catalyst, atmospheric pressure, 60.degree. C., aqueous solution), we obtained less than 2% yields of methoxyacetone (see Comparative Example 3 below). Similar results were observed for air oxidations at high pressure (1000 psi) as shown by Comparative Example 4.
Hydrogen peroxide has been used in a number of liquid-phase oxidation processes. For example, U.S. Pat. No. 4,480,135 teaches that secondary alcohols can be oxidized to ketones using aqueous hydrogen peroxide and a synthetic zeolite containing titanium. Hydrogen peroxide has also been used with a phosphotungstate in a two-phase system (see U.S. Pat. No. 4,754,073). The organic phase contains the secondary alcohol and tungstate, while the aqueous phase contains H.sub.2 O.sub.2. Hydrogen peroxide has apparently not been used to make methoxyacetone.
In sum, an improved process for making methoxyacetone is needed. Preferably, the process could be practiced using common laboratory equipment under mild conditions with readily available reagents. A valuable process would improve on the yield and selectivity of methoxyacetone compared with that available from known vapor and liquid-phase oxidation processes.